A Doppler radar radiates rf energy of approximately 25 dbm and processes a return echo with a minimum signal strength of approximately -145 dbm in a 100 Hz bandwidth. In order to prevent the transmitter from saturating the receiver, or protect the receiver from modulation products of the transmitter due to vibration, etc. which may cause a spurious Doppler return and invalid velocity outputs, an isolation between the transmitter and receiver of approximately 170 db is required. There are Doppler radars that are currently operational that achieve this by using separate transmit and receive antennas. However, this has the disadvantage that each antenna uses only 50% of the available aperture, and therefore has double the beamwidth of an antenna that uses 100% of the aperture. Since the short term error of the velocity output of a Doppler radar is related to the square of the Doppler beamwidth, a separate transmit-receive antenna has obvious performance disadvantages.
Newer Doppler radars use modulation techniques to permit use of a single antenna for transmitting and receiving. Frequency modulation of the rf reference achieves about 110 db of isolation, but this still necessitates post mixer electronic processing to remove an additional 60 db of transmitter-receiver "leakage." Even in practical solid state pulse-coherent Doppler radar configurations where the transmitter and receiver functions are alternated in time at a 50% duty ratio, leakage to noise ratios of greater than 50 db are observed.